The present invention relates to steam turbines and, in particular, to airfoils configured for removing moisture from the steam flow path of a steam turbine.
In the case of wet steam, both steam-borne, or primary, moisture and moisture deposited on the internal metal surfaces of the steam path, i.e. secondary moisture, cause efficiency losses and the potential for erosion. The path of deposited moisture in a steam turbine stage may be tracked as follows. In the context of a steam turbine, a xe2x80x9cstagexe2x80x9d is comprised of and defined as two rows of airfoils; one stationary and the other rotating with the rotating row of airfoils disposed downstream of the stationary row of airfoils. The moisture starts out as either primary moisture or secondary moisture that is carried over from the previous stage or stages of the turbine. With reference to FIGS. 1 and 4, the moisture, shown generally at 10 in FIG. 4, is deposited on the pressure or concave side 12 of the stationary airfoil 14. The moisture is driven by the steam to the stationary airfoil trailing edge 16 as shown in FIGS. 4 and 5. The moisture is torn off from there in the form of clusters of water which move in the same direction as, but slower than the steam, in the wake behind the stationary airfoils 14. The moisture is then atomized as the relative velocity between it and the surrounding steam reaches a certain threshold. At this point, the moisture is significantly increased in its rate of acceleration while still moving slower than the surrounding steam.
Referring to FIG. 2, a velocity vector diagram in the nozzle-bucket space is shown. In this figure, W is the bucket rotational speed, VW is the absolute water velocity, VS is the absolute steam velocity, VSB is the steam velocity relative to the bucket, and VWB is the water velocity relative to the bucket. Thus the moisture, shown generally at 18, at a high relative velocity VWB impacts the rotating airfoil leading edge 22, as shown in FIG. 1. Referring to FIG. 3, conventionally a number of (typically three) radial grooves 24 are located on the rotating airfoil 20 suction or convex side 26, close to the airfoil leading edge 22 for removing the moisture 18 impacted thereon. The disadvantage of these grooves 24 is that they only remove moisture that has already caused significant efficiency losses. Indeed, efficiency losses of various kinds are realized from when the moisture 10 is first deposited on the stationary airfoil pressure side 12 up to and including moisture 18 interception by the rotating leading edge 22.
FIG. 6 is a view similar to FIG. 5, showing the moisture, generally at 28, that is deposited on the pressure or concave side 30 of the rotating airfoil 20. The moisture is driven by the steam to the rotating airfoil trailing or exit edge 32 and, at the same time, by centrifugal force to the bucket tip 33. From there, part of the secondary moisture is removed from the steam flow path, while the remainder is carried over to the next stage, with the potential for further efficiency losses and erosion.
To remove moisture from the steam flow path in a steam turbine, a single groove or a plurality of grooves are located on the pressure or concave side of nuclear and fossil steam turbine stationary and rotating airfoils operating in a wet steam environment. In the presently preferred embodiment, these grooves are oriented predominantly radially and are located closer to the airfoil trailing or exit edge than the leading or inlet edge. These grooves raise the effectiveness by which secondary moisture is removed from the steam path and eliminate or reduce the efficiency losses associated with this moisture.
Accordingly, the invention is embodied in an airfoil for removing moisture from the flow path of a steam turbine wherein the airfoil has first and second longitudinal ends and an outer peripheral wall extending therebetween. The outer peripheral wall defines an airfoil leading edge, an airfoil trailing edge, a generally concave side face and a generally convex side face. At least one groove is defined in the concave side face so as to extend in a direction generally corresponding to a longitudinal extent of the outer peripheral wall along at least a portion of a length of the outer peripheral wall.
The invention is also embodied in a steam turbine having at least one row of stationary airfoils and at least one row of rotating airfoils disposed downstream from said stationary airfoils, wherein the airfoils of at least one of those rows are moisture removing airfoils, each moisture removing airfoil having first and second longitudinal ends and an outer peripheral wall extending therebetween, the outer peripheral wall defining an airfoil leading edge, an airfoil trailing edge, a generally concave side face and a generally convex side face, and wherein at least one groove is defined in the concave side face, the groove extending along at least a portion of a length of the outer peripheral wall.